This invention relates generally to portable supports and more particularly to such supports suitable for holding surveying equipment and the like.
Although the art of surveying is old, recent improvements in equipment, such as automatic electronic total surveying stations, have increased the accuracy of the surveying instruments far beyond what was possible previously. Where accepted accuracy for equipment in the not too distance past was 20 seconds of a degree, accuracy is now commonly required to be 1 second of a degree. At these levels of precision, the instruments must be supported in a manner which is consistent and stable. Accordingly, there is a need for closer examination of support structure used for surveying equipment so that improvements in instrumentation will not be lessened by inadequate supports.
A most common support for surveying equipment is the tripod with telescoping legs that can be retracted for carrying and storage. Certainly bipods are also employed in surveying, and conceivably any number of legs could be provided, but the three point support remains the most common. Frequently, surveying is done far from roads or sidewalks so equipment may have to be carried over broken terrain for considerable distances to reach the site. As a result, it is imperative that the tripod be able to collapse for easy transportation. However, the ability to collapse can be the source of imprecision in supporting the equipment when the site is reached. When the telescoping legs are extended, it is necessary that they be locked in place. Movement thereafter, even on a scale which would not be readily perceptible to the surveyors, can detrimentally affect the accuracy of the supported equipment. Presently, locking devices used for locking the legs in a fixed position of extension are clamps which apply a clamping force to raise the static friction between the leg members over a highly localized area. To hold the leg members securely a substantial force is required. Tripods having metal legs are usually able to withstand the force, but are capable of plastic deformation over time so that there is a loss of clamping force. Lighter weight, non-metallic materials do not work well at all with highly localized application of the clamping force, making it more difficult to use these materials. Often a second clamp is used to provide the clamping force in two locations.
A typical construction of a telescoping leg may have a center extensible leg member and two fixed leg members, in the form of rods, on opposite sides of the center leg member. Rounded channels are provided in the sides of the center leg member which receive the round rods, connecting the leg members together for relative sliding motion. The rods are squeezed into respective channels to lock the leg members in a selected position of extension. However, in practice the contact between each rod and the center leg member in the channel is frequently only along a line. In practice, line contact substantially limits the static friction between the rods and the center leg member even when a large clamping force is applied, making it more likely that some small movement may occur even after the leg members are locked in position.
Problems with accuracy and stability in positioning instrumentation may also arise from minor damage to the tripod which may occur when the tripod is being stored or transported. The rods are held at their upper ends in a hinge which attaches the rods to a head of the tripod on which equipment is mounted. The hinge permits the legs to be pivoted between a use position in which the legs are separated apart and a collapsed position in which the legs are close together, usually contacting each other. The rods are bolted or otherwise fixed to the hinge at spaced apart locations so that the hinge tends to act like a parallelogram linkage when lateral forces are applied to the hinge. There is a tendency for the rods to tilt into a parallelogram configuration, perhaps becoming permanently deformed in this configuration. A similar thing may happen when motorized equipment mounted on the head turns, applying torsional load to the leg.
Among the several objects and features of the present invention may be noted the provision of a support for surveying equipment which provides a stable and rigid platform for high precision surveying equipment; the provision of such a support which tightly locks telescoping leg members of the support in a selected position of extension; the provision of such a support which requires minimal clamping force to achieve the leg member lock; the provision of such a support which helps to keep the clamp from sticking in a locked position; the provision of such a support which actively operates to unlock the leg members when the clamping force is released; the provision of such a support which can be made of lighter weight material; the provision of such a support which is made of material which is not subject to plastic deformation; the provision of such a support which has legs constructed to resist lateral loads; the provision of such a support which is easy to transport; the provision of such a support which can be manufactured with high precision; and the provision of such a support which can be manufactured economically.
Further among the several objects and features of the present invention may be noted the provision of a method of assembling a leg for a portable support which produces a strong, lightweight leg; the provision of such a method which repeatedly produces high precision in the assembled leg; the provision of such a method which can be rapidly carried out in assembly line production; the provision of such a method which produces a unitary structural unit; and the provision of such a method which can be carried out economically.
Generally, a portable support for use in supporting equipment above a surface comprises a head for holding the equipment and legs connected to the head for supporting the head above the surface. Each leg comprises a fixed leg member connected to the head and a telescoping leg member connected to the fixed leg member for sliding motion relative to the leg member between a fully extended position and a fully retracted position. A clamp has a locked position for squeezing the telescoping leg member and the fixed leg member together to secure the telescoping leg member in a selected position relative to the fixed leg member, and an unlocked position in which the telescoping leg member is movable relative to the fixed leg member. The fixed leg member and the telescoping leg member of at least one of the legs each including a generally wedge-shaped engagement formation interengageable with the wedge-shaped engagement formation of the other under the clamping force of the clamp in the locked position to hold the telescoping leg member in a fixed position with respect to the fixed leg member. The wedge-shaped formations being sized and shaped to prevent the formations from bottoming out in the locked position of the clamp.
In another aspect of the present invention a leg for a support as described above.
In a further aspect of the present invention, a leg for a portable support for use in supporting equipment above a surface generally comprises a fixed leg member and a telescoping leg member connected to the fixed leg member for sliding motion relative to the leg member between a fully extended position and a fully retracted position. The fixed leg member comprises a pair of rods disposed on opposite sides of the telescoping leg member and a rod receptacle comprising a housing, a locating block, a fastener for securing the rods against the locating block in a relaxed position in which the rods are substantially stress-free, and adhesive in the housing substantially filling spaces in the housing. A clamp has a locked position for deflecting the rods inwardly from the relaxed position toward the telescoping member to selectively secure the telescoping leg member in a selected position relative to the fixed leg member, and an unlocked position in which rods are released to their relaxed positions and the telescoping leg member is movable relative to the rods.
In a still further aspect of the present invention, a method for making a leg for a portable support generally comprises the steps of placing the ends of two rods in a housing and locating the rods against a locator block on opposite sides thereof such that the locator block fixes the separation of the rods. Adhesive is injected into the housing around the rods and block to substantially fill the housing whereby the rods, locating block and housing constitute a substantially unitary structure. A telescoping leg member is fitted between the rods such that the telescoping leg member is capable of sliding longitudinally of the rods for selective adjustment of the length of the leg.
In yet another aspect of the present invention, a leg for a portable support for use in supporting equipment above a surface generally comprises a fixed leg member and a telescoping leg member connected to the fixed leg member for sliding motion relative to the leg member between a fully extended position and a fully retracted position. The telescoping leg member has a front face directed generally outwardly of the support and a rear face. The telescoping leg member is connected to the fixed leg member so that the telescoping leg member is free of obstructions at least at one point on the front face at all locations of the telescoping leg member between the filly extended and fully retracted positions.
In a further aspect of the present invention, a portable support for use in supporting equipment above a surface generally comprises a head for holding the equipment and legs connected to the head for supporting the head above the surface. Each leg comprises a fixed leg member connected to the head and a telescoping leg member connected to the fixed leg member for sliding motion relative to the leg member between a fully extended position and a fully retracted position. A clamp has a locked position for squeezing the telescoping leg member and the fixed leg member together to secure the telescoping leg member in a selected position relative to the fixed leg member, and an unlocked position in which the telescoping leg member is movable relative to the fixed leg member. The clamp comprises a pair of opposed jaws located on opposite sides of the leg and a connecting bar slidably connecting the jaws together. A cam pivotally mounted on the connecting bar and engageable with one of the jaws is operable to push the jaws toward each other on the connecting bar in the locked position and to permit the jaws to move apart in the unlocked position. At least one spring release element associated with one of the jaws between the jaw and the leg is compressed against the leg when the clamp is in the locked position. The spring element actively pushes the jaws apart when the cam permits the jaws to move apart.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.